Ok, I'm writing a story where the character visits a museum. Does she visit:
- the mammal exhibit
- the mammals exhibit
- the mammals' exhibit
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the mammal exhibit
Mammal is often used in this way. See the Oxford English Dictionary. I quote the adjectival definition in full in case you can't access the OED.
(attrib.). Of, relating to, or of the nature of a mammal or mammals.
1840 C. S. Rafinesque Amenities of Nature in Good Bk. No. 1. 9 The sucklers or mammal animals.
1844 R. Chambers Vestiges Nat. Hist. Creation 201 In the mammal fœtus,..the organ has the form of a prolonged tube.
1879 tr. Haeckel Evol. Man I. i. 3 Amphibian and Mammal forms.
1902 Pop. Sci. Monthly Sept. 436 A whale may be alluded to as a gigantic mammal or a mammal giant.
1957 P. J. Darlington Zoogeogr. vii. 469 A small fraction of the Oriental mammal fauna extends for a considerable distance across Wallace's Line.
1988 Conservation Biol. 2 336 Species richness of small mammal communities..directly correlated with shelterbelt length.
1994 Nature 24 Mar. 298/2 This island-continent has until now yielded no confirmed pre-Holocene mammal fossils.
Similarly, your character might visit the reptile house at the zoo. In his comment, below, @sumelic gave a technical explanation of this usage.